1. Field of the Invention
The present invention relates to a signal read out circuit to read out a plurality of signals to a signal line.
The invention also relates to a photoelectric converting apparatus and, more particularly, to a photoelectric converting apparatus formed on a semiconductor substrate.
2. Related Background Art
FIG. 6A is a circuit diagram showing an example of a conventional signal read-out; and circuit.
In the diagram, after a plurality of signals S.sub.1 to S.sub.n are accumulated in capacitors C.sub.1 to C.sub.n having a capacitance C.sub.t, they are sequentially read out to a signal line 401 through switching transistors Q.sub.1 to Q.sub.n and output from an amplifier 403. The switching transistors Q.sub.1 to Q.sub.n are turned on or off by scan pulses .0..sub.h1 to .0..sub.h2 which are output from a scan circuit 402.
However, the conventional circuit has the following problems.
(1) Since the signal line 401 has a stray capacitance C.sub.h, when the signals are transferred from the capacitance C.sub.t of the capacitors C.sub.1 to C.sub.n to the capacitance C, the signal level of the signal line 401 is decreased by capacitance division to a value which is C.sub.t /(C.sub.t +C.sub.h) times as low as the level when the signals were accumulated in the capacitors.
In particular, in the case of a high density sensor or the like which has a large number n of signals, the capacitance C.sub.h of the signal line 401 increases, so that the output remarkably decreases.
(2) As a method of preventing the decrease in output, it may be considered to enlarge the capacitance C.sub.t, but this results in an increase in load capacitance of the sensor where signals S.sub.1 to S.sub.n are the outputs of the sensor. There then occurs a new problem such that the speed of transfer of a sensor signal S to the capacitors decreases and a high speed operation cannot be performed.
On the other hand, in the conventional photoelectric converting apparatuses using such a reading circuit, there is also a photoelectric converting apparatus in which the signals generated and accumulated in the photoelectric converting areas are transferred to a first capacitive area such as a vertical line or the like provided separately from the photoelectric converting areas. Then the signals accumulated in the first capacitive area are transferred to a second capacitive area such as a horizontal output line or the like, and the signals accumulated in the second capacitance area are output. An example of such a conventional photoelectric converting apparatus will be explained with reference to FIG. 6B.
FIG. 6B is a circuit diagram of a photoelectric converting apparatus in which fundamental photosensor cells 30 are two-dimensionally arranged in a matrix form of 3.times.3.
In FIG. 6B, reference numeral 30 denotes the fundamental photosensor cells which are constituted in the following manner. The carriers generated in the PN junction portion of a bipolar transistor by light excitation are accumulated in a base region. An output line connected to an emitter region is set in the floating state. By applying positive pulses to the base through a capacitor connected thereto, the carriers accumulated in the base region are read out so as not to be destroyed. The output line connected to the emitter region is, for example, grounded. By applying positive pulses to the base through the capacitor, the carriers accumulated in the base region are erased. The photoelectric converting apparatus in FIG. 6B will now be described with reference to the diagram. The conventional photoelectric converting apparatus in FIG. 6B comprises horizontal lines 31, 31', and 31" to apply readout pulses and refreshing pulses; a vertical shift register 32 to generate the readout pulses and refreshing pulses; a terminal 34 to apply gate pulses to buffer MOS transistors 33, 33', and 33" which are arranged among the vertical shift register 32 and the horizontal lines 31, 31', and 31"; vertical lines 38, 38', and 38" to read out the accumulated voltages from the fundamental photosensor cells 30; a horizontal shift register 39 to generate pulses to select each vertical line; MOS transistors 40, 40', and 40" for gates to open or close each vertical line; a horizontal output line 41 to read out the accumulated voltages to an amplifying section; a MOS transistor 42 to refresh the charges accumulated and held in the output line 41 after they were read out; a terminal 43 to apply the refreshing pulse to the MOS transistor 42; a transistor 44 such as bipolar transistor, MOS-FET, J-FET, or the like to amplify the output signal; a load resistor 45; a terminal 46 to connect the transistor with a power source; an output terminal 47 of the transistor; MOS transistors 48, 48', and 48" to refresh the charges accumulated in the vertical lines 38, 38', and 38" in the reading operation; and a terminal 49 to apply pulses to gates of the MOS transistors 48, 48', and 48". In such a photoelectric converting apparatus, a pulse is first applied to the terminal 49, thereby turning on the MOS transistors 48, 48', and 48". The vertical lines 38, 38', and 38" are previously grounded and cleared. Next, the MOS transistors 48, 48', and 48" are turned off. Pulses are applied through the MOS transistors 33, 33', and 33" to the horizontal lines 31, 31', and 31" selected by the vertical shift register 32. The signals of the photosensor cells 30 are read out to the vertical lines 38, 38', and 38" in the floating state. The vertical lines 38, 38', and 38" have a peculiar capacitive component. The signals corresponding to the signals of the photosensor cells are held in the capacitance of the vertical lines by the reading operations. Next, the MOS transistors 40, 40' and 40" are then sequentially selected by the horizontal shift register 39. The signals held in the peculiar capacitance of the vertical lines 38, 38', and 38" are applied through the horizontal line 41 to a control electrode of the transistor 44. The signals corresponding to the outputs of the photosensor cells 30 are sequentially output from the terminal 47.
On the other hand, when the pulses are being applied through the capacitors connected to the bases of the photosensor cells 30 after that, if the pulses are applied to the terminal 49 and the vertical lines 38, 38', and 38" are grounded, the carriers accumulated in the base regions can be erased.
Even in the foregoing conventional photoelectric converting apparatus, when the signals held in the peculiar capacitance of the vertical lines 38, 38', and 38" are sequentially applied through the horizontal line 41 to the control electrode of the transistor 44, the level of the signal which is applied to the transistor 44 is determined by the ratio of the peculiar capacitance of the horizontal line 41 and the peculiar capacitance of the vertical lines 38, 38', and 38" which are accessed by the horizontal register 39. Thus, this signal level decreases in dependence on the dividing ratio of the capacitance.
Such a reduction in signal level still becomes typical with an increase in the number of horizontal pixels. This is because the capacitance of the horizontal line 41 substantially increases in proportion to the number of MOS transistors 40, 40', 40", . . . for gating.
Therefore, to avoid the reduction in signal level, it is necessary to further increase the capacitance of the vertical lines or to use a multi-output line system in which the horizontal line is divided and the signals are read out.
However, there are drawbacks and problems such that the former method causes an increase in chip area and the latter method causes an increase in the number of output terminals, i.e., number of pins.
On the other hand, as another method, there may be considered an idea such that by inserting an amplifier between the vertical line and the horizontal line, the reduction in signal level is prevented. However, this method has a drawback such that since a constitution of the amplifying section is complicated, the chip area increases and a constitution of the horizontal shift register section becomes further complicated